The phase diagram of lithium niobate single-crystal was known from long ago. For producing lithium niobate single-crystal with high compositional homogeneity, one conventional method known in the art comprises rotational pulling of crystal from a flux as combined with growing the crystal being pulled, in which the flux has a congruent melt composition of such that the crystal being grown and the flux are equilibrated to have the same composition, and has a molar fraction of Li.sub.2 O/(Nb.sub.2 O.sub.5 +Li.sub.2 O) of being 0.485. Since the as-grown lithium niobate single-crystal thus produced in the method is in a multi-domain condition, it is subjected to poling treatment of applying a voltage thereto in the direction of the Z-axis of the crystal being heated at a temperature not lower than its Curie temperature of about 1150.degree. C. to thereby unipolarize the crystal, followed by cooling it. Then, the resulting, single-domain crystal is worked to have a predetermined size and used in various fields.
As having a favorable electrooptical constant and a favorable non-linear optical constant, the lithium niobate single-crystal is widely noticed as a substrate material for light modulators, light switches, Q switches, wave changing devices, etc. Recently, in particular, expected is the development of waveguide-type, optical second-harmonic generation (SHG) devices capable of converting a semiconductor and solid state lasers having a near infrared wavelength into a blue light having a semi-wavelength by means of a non-linear optical effect. Above all, most studied are SHG devices comprising an element of lithium niobate single-crystal with its polarization structure being periodically inverted, as light sources for high-density recording and reproduction of optical discs. The SHG devices of that type are driven in a quasi-phase matching (QPM) system, in which the difference between the propagation constant of the fundamental wave and that of the higher harmonic wave is compensated for by the periodic structure to gain the phase matching. This system has many excellent characteristics in that its conversion efficiency is high, that parallel beaming and diffraction-limited collection of the light being outputted therefrom is easy, and that there is no limitation on the applicable material and wavelength. As the periodic structure for QPM, a structure of which the SHG coefficient (d coefficient) attribute is periodically inverted is most preferred for obtaining a high efficiency, and the positive or negative attribute of the d coefficient of ferroelectric crystal corresponds to the polarity of the ferroelectric polarization of the crystal. Therefore, the periodically-inverting structure of polarized ferroelectric domains is used in the QPM system. In the QPM-SHG system, usable are non-linear optical constants d22 and d33, which, however, could not be used in a phase matching system based on birefringence, and the QPM-SHG system of that type has the great advantage of high-efficiency wavelength conversion.
As compared with any other non-linear, optical single-crystal, lithium niobate single-crystal has a large non-linear optical constant (d33 of 34.4 pm/V), and this is one of materials which have heretofore been most studied for producing optical devices comprising them. The most important technique for realizing QPM-SHG devices comprising ferroelectric crystal is to produce periodic polarization-inversion domains with accuracy. The phase matching period around the fundamental wavelength, 0.8 .mu.m of lithium niobate single-crystal is about 3 .mu.m or so. However, the single-domain LN (lithium niobate) single-crystal as prepared by poling the as-grown one is extremely stable around room temperature, and it is not easy to invert the polarity of the crystal in an ordinary electric field. In this connection, reported were some techniques for polarization inversion of LN single-crystal at a temperature not higher than the Curie point of the crystal by various methods. The reported methods include, for example, 1) internal Ti diffusion, 2) SiO.sub.2 -charged thermal treatment, 3) proton-exchanging thermal treatment, 4) electron beam-scanning irradiation, and 5) voltage application. There are known many reports referring to the voltage application method 5). In one report, a periodic electrode is provided on one surface of a Z-cut LN single-crystal substrate, while a uniform electrode on the other surface thereof, and a pulse voltage is imparted to the crystal substrate via those electrodes to thereby obtain periodic polarization inversion of nearly the same pattern as that of the periodic electrode. By applying a near infrared laser to the QPM-SHG device thus produced in that manner, obtained is a blue SHG laser ray of a few mW or so. Except for SHG, QPM devices comprising LN single-crystal are further studied for application to wavelength conversion systems such as near infrared OPO, etc.
As has been mentioned hereinabove, the most important technique for realizing QPM-SHG devices comprising ferroelectric single-crystal is to produce periodic polarization-inversion domains with accuracy. Ideally, it is important to enlarge the overlapping of the inverted structure with the guided wave mode and to reduce the normalization matching error, or that is, to gain the polarization to inversion width ratio of 1/1. In fact, however, since the tolerance for the QPM condition is very narrow, the inversion period insufficiency, if any, in the devices produced ends in failure in realizing small-sized, high-efficiency devices. The method of electronic beam-scanning irradiation or voltage application to lithium niobate crystal for the polarization inversion of the crystal will be advantageous in that inverted lattices which are nearly uniform in the direction of the thickness of the crystal are formed. Even in this, however, it is still extremely difficult to gain the polarization to inversion width ratio of being completely 1/1. In addition, the process itself is problematic in its reproducibility. For example, in the voltage application method, a periodic electrode is provided on one surface of a Z-cut lithium niobate single-crystal substrate, while a uniform electrode on the other surface thereof, and a pulse voltage is imparted to the crystal substrate via those electrodes to thereby polarize and invert the area of the crystal substrate just below the periodic electrode, in the direction of the Z-axis of the crystal. In this, however, the inverted and polarized width of the crystal does not always correspond to the electrode width, and, in addition, the production error is great. Moreover, the method is further problematic in that the inversion will be often stopped in the middle of the formation of polarization and inversion on the Z-axis direction on the opposite surface of the crystal substrate, and that the polarized and inverted width will differ between the both surfaces of the Z-cut crystal substrate. For these reasons, therefore, it is difficult to produce ideal QPM-SHG devices according to this method.
The periodic width of polarization and inversion varies, depending on the phase-matching wavelength of the intended SHG device. For example, for long-wave phase matching, for example, in OPO devices, the inversion width to be controlled is large to be over ten .mu.m or so. Therefore, the formation of long-wave devices is relatively easy as compared with that of short-wave ones in which the inversion width to be controlled is about 3 .mu.m or so. However, the conventional methods could not still realize the production of ideal SHG devices. On the other hand, LN single-crystal requires a high voltage of not lower than 20 kV/mm for its polarization inversion. For a thin substrate of LN single-crystal having a thickness of 0.5 mm or so, it may be possible to produce polarized and inverted lattices throughout the entire substrate. However, thick substrates of LN single-crystal having a thickness of a few mm or so are problematic in that the production of complete polarization and inversion therein is difficult.